An automatic gain controller is often used with amplification circuitry to control the amplitude of output signals from amplification circuitry. In radio communication circuitry, communication signals are received via a receiver, and then amplified for use in signal processing. Signals received by the receiver may be of varying signal strength. If amplified without automatic gain control (AGC), the resultant signals may have too much or too little gain, with a corresponding fluctuation of signal amplitudes. Variations in signal amplitude may cause significant problems in circuitry attempting to recover information transmitted in the received signals. Accordingly, an AGC circuit is usually employed in an effort to present a signal with relatively constant amplitude.
A typical AGC circuit detects amplitude changes in a signal to be amplified and controls the gain of the amplification stage such that if the input signal strength diminishes, the amplification gain is increased, and if the input signal strength increases, the gain of the amplification stage decreases. Thus, the automatic gain controller attempts to control the output of the amplification stage such that a signal having a relatively constant amplitude is outputted. This type of AGC is common in communication circuitry.
FIG. 1 is a block diagram 10 showing a prior art automatic gain control circuit 30 for controlling the gain of an amplifier to control the amplitude of the output signal from the amplifier. An input signal 25 is coupled to an input port 21 of a variable amplifier 20 which outputs an amplified signal 27 through an output port 22. A signal energy detector 31 is coupled to the output signal 27 to detect the information relating to the amplitude of the output signal 27. A comparator 32 takes the output of the signal energy detector 31, and compares it against some desired reference value, such as a reference voltage. Depending on the comparison results, a feedback signal generator 33 generates a feedback signal 37 which is coupled to a port 23 on the amplifier 20 to vary the gain of the amplifier 20. Such automatic gain control circuitry 30 in various forms is typical in the art.
A major problem which exists with such automatic gain control circuitry lies in the difficulty of maintaining precise control of a target amplitude level for an output signal. A significant aspect of the problem is the decision making process for determining when to make adjustments to the amplification gain. In a typical prior art automatic gain control circuitry, the signal energy detector is implemented using a rectifier and a low-pass filter which provides an average of the energy content of the signal being monitored. Generally, it is difficult to determine whether energy fluctuations detected by the low-pass filter is a result of changes in amplitude of the signal being processed, or changes in the frequency or quality of such signals. Such detail information is lost in the averaging process performed by the low-pass filter typically used. The consequence is that unnecessary adjustments may be made in the gain of the amplifier in response to changes in the signals that have nothing to do with peak amplitude or signal strength. The inability to properly and correctly characterize the peak amplitude of this signal is a problem generally encountered in prior art AGC circuits. For example, if the received signal becomes noisy, additional energy may be detected by the low-pass filter of the AGC, and an undesirable response of amplification of the noisy signal may result.
The lack of precise control offered by conventional AGC circuits is a serious deficiency in the prior art. Generally, prior art AGC circuitry responds to distortions or disruptions to the signal being processed, however caused, and may make inappropriate changes to the amplification gain of signals. In circuits requiring precise control of peak signal amplitude, such average energy AGC circuitry proves inadequate. Thus, it is desirable to have an improved apparatus and method for precisely controlling gain and/or peak amplitude of signals in an amplification circuit. dr